The 60-year-old United Nations complex, which almost entirely lacks sprinklers, blastproof windows, and many essentials of contemporary diplomatic architecture, does have lots of one amenity that its designers considered indispensable: ashtrays. In the lobby of the U.N.’s most visible building, the Secretariat tower, lovingly designed geometric cigarette receptacles protrude from green marble walls. In the great public rooms, pairs of vinyl-covered chairs are joined by steel bowls that haven’t seen ash in years.

The original trinity of U.N. buildings—the Secretariat, the low-slung swoop of the General Assembly, and the boxy conference building—is a decaying time machine. Rain seeps around ancient windows and leaches asbestos from crumbling ducts. Cumbersome fire doors break up the once-flowing hallways. An improvised tent for metal detectors vitiates any sense of grandeur in the approach. Where once the architecture promised a more perfect world, now it has clearly seen better days.

Renovation has begun. Already, the horseshoe desk is gone from the Security Council chamber, leaving a concrete husk where Colin Powell once detailed Saddam Hussein’s nonexistent weapons of mass destruction. Soon the Secretariat will shed its skin of blue-green glass, which in certain light looks made of seawater. The deliberate devastation will continue for a couple of years, as heads of state convene in a makeshift warehouse on the campus’s north lawn. Yet as the technology gets brought up to date, this period piece of mid-century modernism—shrine to a time when discussions about genocide and hunger took place amid eddies of cigarette smoke—will be meticulously reassembled. “We will keep the old ashtrays and make them into flowerpots, and we’ll take the Naugahyde and Formica and restore them. We’ll remove 60 years of nicotine stains from the wood,” says Michael Adlerstein, the assistant secretary general in charge of the renovation.

You can’t change a lightbulb in the General Assembly without smacking into modern architecture’s preservation paradox: By the time a building is significant enough to be saved, its significance has irrevocably changed. Structures that once embraced the future become museums of obsolescence. Icons of efficiency are treasured despite their impracticality. At the U.N., these contradictions are exaggerated because its then-fresh form of architecture crystallized immense global ambitions. Now the same design enshrines a murkier history of accomplishments and disappointments.

Among the United Nations’ first tasks—before the partition of Palestine or the Universal Declaration of Human Rights—was to shop for a home, and it found one on a derelict stretch of slaughterhouses and slums along the East River. In 1947, the organization summoned architects from around the world. The pragmatic New Yorker Wallace K. Harrison found himself placating squabbling visionaries. Le Corbusier and Oscar Niemeyer, in particular, fought over the shape, number, and orientation of the buildings, whether the glass curtain wall should include a sun-shading grid of stone, and—most ferociously—who got credit for what. Le Corbusier complained to his mother of the “apparent kidnapping of [his] U.N. project by USA gangster Harrison.” The cocktail of haste, diplomacy, vanity, and genius might have yielded an architectural hangover. Instead, it produced the first monument of postwar modernism.

One thing they and their clients could agree on was that a new world should adopt a new architecture, glittering with innocence, transparency, and logic. This emblem of international comity was also a monument to paper-shuffling, which was precisely the point. The Secretariat is a hive of functionaries, a vast receptacle of diplomatic industry, and it projects a clear and simple message: Peace takes work. Many critics found the complex soulless and its creators deluded in the idea that World War III could be averted by orderly administration. “If the Secretariat Building will have anything to say as a symbol, it will be, I fear, that the managerial revolution has taken place and that bureaucracy rules the world,” wrote Lewis Mumford in The New Yorker.

All these decades later, the U.N. might have decided to rebrand itself, tearing the whole thing down and erecting a glittering new palace. Or it could have crammed into the existing campus a gaudy new addition designed by an architect of suitably global stature. The organization is in New York but not of it; as sovereign territory jointly owned by 192 member states, the buildings aren’t bound by the city’s zoning code or landmark laws. The U.N. could have reshaped a significant chunk of Manhattan without so much as a phone call to the mayor.

Fortunately for New York, an organization founded to change the world has become unbudgeably conservative, and so it will restore the telegenic public spaces and legend-laden halls. A pair of Brazilian murals in the delegates’ lobby will be shipped home to be cleaned. The shapely blond-wood kiosk in the visitors’ lobby will be sent out to a furniture restorer. Craftsmen will build new copies of the colored steel boxes that enclose the ceiling lights in the Trusteeship Council Chamber. New signs will get fifties-style lettering to blend with the old.